The invention relates to the field of telecommunications by optical fiber, and more particularly to telecommunications over long distances. For very long distance optical fiber links, such as transoceanic links, it is known to use a signal of the so-called xe2x80x9csolitonxe2x80x9d type having special spectral properties that enable the signal to propagate along the dispersive fiber without appreciable chromatic dispersion, i.e. advantage is taken of the way refractive index depends on the intensity of the signal to counterbalance chromatic dispersion, or vice versa. The spectral shape of the signal is preserved in spite of the effects of propagation distance, which are thus mainly concerned with line losses. Line losses can be compensated by in-line optical amplification, e.g. by means of erbium-doped fiber amplifiers (EDFAs).
When transmitting solitons with in-line amplification (EDFA) the problems that remain to be solved are known:
1) Gordon-Haus jitter which causes uncertainty concerning the arrival times of the bits of the signal; and
2) the accumulation of noise that comes from the optical amplifiers amplifying spontaneous emission.
One solution to that problem is put forward in EP-A-6 576 208. In that document, a plurality of filters having various center frequencies are inserted at points along a link for transmitting solution type signals, thereby enabling the solitons to be amplified periodically without amplifying spontaneous emission noise exponentially. In that system, the solitons are not regenerated. According to that document, an advantage of such a system is that it is compatible with transmission in the form of a wavelength division multiplex (WDM).
Synchronous modulation for regenerating solitons in-line is described in the document: xe2x80x9c10 Gbit/s soliton data transmission over one million kilometersxe2x80x9d by Nakazawa et al., published in Elect. Lett., 27 (14), pp. 1270-72, Jul. 4, 1991.
That document teaches using an LiNO3 optical modulator to perform synchronous modulation of solitons, with a clock signal generated using the same clock as that used for the soliton source. A very long distance link was simulated using a 500 km long fiber loop with an erbium-doped filter optical amplifier every 50 km and with regeneration once per trip round the loop. Because of the dispersion of the soliton transmission fiber, which lies in the range xe2x88x920.7 ps/km/nm to xe2x88x922.2 ps/km/nm with a mean value of xe2x88x921.5 ps/km/nm, the travel time for making one trip round the loop depends on the wavelength of the soliton. That is why such a system is incompatible with WDM transmission, as emphasized in the above-mentioned document EP-A-0 576 208 (cf. page 2, lines 21-24).
Other documents in the state of the art relate to WDM type optical links.
For example, the document xe2x80x9cWavelength division multiplexing with solitons in ultra-long distance transmission using lumped amplifiersxe2x80x9d by L. F. Mollenauer et al., published in Journal of Lightwave Tech., 9 (3), pp. 362-367, March 1991, proposes a system for transmitting WDM solitons with periodic optical amplification for transoceanic distances (9000 km). The teaching of that document relates mainly to collisions between solitons of different wavelengths. That document gives typical values for various parameters in such a link so as to limit the Gordon-Haus jitter that stems from interaction between adjacent channels. Nevertheless, in all the cases considered in that document, synchronous arrival of solitons at the end of the link is neither provided nor required.
The various aspects of managing dispersion in non-regenerated WDM systems are also considered in the document xe2x80x9cDispersion managements on soliton transmission in fibers with lumped amplifiersxe2x80x9d by S. Kumar et al., published in Proc. Int""l. Symposium on Physics and Applications of Optical Solitons in Fibers, Kyoto, Japan, pp. 1-12, Nov. 14-17, 1995 (cf. last chapter of the document).
Thus, on reading the documents of the prior art, it can be seen that optical links having a plurality of wavelength division multiplexed channels do not enable solitons to be regenerated since the channels are not synchronous. In this context, the question of synchronous regeneration is therefore beside the point.
That is why, starting from well-established prejudices of the person skilled in the art it would not appear possible to envisage very high data rate WDM optical links over very long distances using wavelength division multiplexed solitons and regeneration for eliminating Gordon-Haus jitter and for maintaining the optimum spectral shape of the solitons.
An object of the invention is to mitigate the drawbacks of the prior art.
To this end, the invention provides apparatus for regenerating an optical signal in the form of a bit stream represented by solitons defined in particular by a propagation wavelength and a bit rate, said apparatus comprising a clock recovery circuit for extracting a clock signal from said optical signal and an optical modulator for regenerating said solitons, and being characterized in that it includes, upstream from the modulator, synchronization means for synchronizing solitons emitted on n channels having respective different wavelengths, where n greater than 1, said channels and said different wavelengths being associated with different group times, said synchronization means having m optical delay lines, where 1xe2x89xa6mxe2x89xa6n, the delay xcfx84i for the line i, where 1xe2x89xa6ixe2x89xa6m, being selected in such a manner as to compensate for the differences between the group times associated with the various channels.
In an advantageous embodiment, the synchronization means have m optical delay lines, where mxe2x89xa6n, the delay xcfx84i for channel i, where 1xe2x89xa6i less than m, being selected in such a manner as to compensate for the group time differences between m channels, and also have at least one line without optical delay for the n-m other channels.
In particular, the synchronization means comprise a single line without optical delay, said line without optical delay being designed to receive multiplexed solitons emitted over a plurality of channels.
In a presentlt preferred, first embodiment, the synchronization means includes an optical line fitted with m photorefractive filters in series, the frequency of each filter being associated with the frequency of a respective channel, and the respective position of each filter i, where 1xe2x89xa6ixe2x89xa6m, being selected so as to produce said delay xcfx84i for the solitons emitted on channel i; control means for applying the solitons received by the synchronization means to said optical line and for applying the solitons reflected by the filters of said optical line to an outlet port of the synchronization means; and an optical coupler for conveying the solitons emitted on the n-m channels which are not associated with a filter to the outlet port of the synchronization means. In this embodiment, the control means is a three-part optical circulator.
In a second embodiment of the invention, the synchronization means comprises a demultiplexer, a set of m lines in parallel, each including a length of optical delay line, a multiplexer, and at least one line without optical delay disposed between the demultiplexer and the multiplexer.
In a third embodiment of the invention, the synchronization means comprise: a divider; a set of m lines in parallel each having a respective filter for selecting one channel, and a length of optical delay line; a concentrator; and at least one line without optical delay between the divider and the concentrator, said line without delay having a filter for selecting at least one channel.
The invention also provides an optical transmission system for conveying signals each of which is in the form of a bit stream represented by solitons, which solitons are defined in particular by a propagation wavelength and by a bit rate, said transmission system comprising at least an emitter and a receiver interconnected by an optical fiber, said system including at least one optical regenerator apparatus of the invention.
Advantageously, in such an optical transmission system, each regenerator apparatus is disposed at a distance ZR from said emitter or from the preceding regenerator apparatus, where the distance ZR is selected in such a manner that its product with the arrival time difference xcex4xcfx84g=xcfx84g(xcex1)xc2x7xcfx84g(xcexl) satisfies the following condition:
[kTxe2x88x92T/a] less than xcex4xcfx84gZR less than [kT+T/a]
where: k is an integer; axe2x89xa74; T is the bit time; ZR is in km; dtg is in ps.kmxe2x88x921; and xcex1 and xcexl are the end wavelengths of the spectrum band defined by said subset of n-m channels.
In a particularly advantageous manner, the clock recovery circuit extracts from the optical signal a signal of wavelength xcexk lying in the range xcex1 to xcexl, such that xcfx84g(xcexk).ZR=kT.